1 a. Design of hording structures.pptx

Hoarding Structures
• Analysis and design of hoarding
structures under dead, live and wind
load conditions as per codal provisions
by limit state method
• Introduction to fatigue failure.
1

5.3: Design Wind Speed (Vz)
The basic wind speed for any site shall be
modified to include the following effects to get
design wind speed, Vz at any height, z for the
chosen structure: (a) Risk level, (b) Terrain
roughness and height of structure, (c) Local
topography, and (d) Importance factor for the
cyclonic region. It can be mathematically
expressed as follows.
6

Vz = Vb k1 k2 k3 k4, where
Vz = design wind speed at any height z in m/s
k1 = probability factor (risk coefficient) (see 5.3.1)
k2 = terrain roughness and height factor (see 5.3.2)
k3 = topography factor (see 5.3.3)
k4 = importance factor for the cyclonic region (see
5.3.4).
Note: The wind speed may be taken as constant up
to a height of 10 m. However, pressures for
buildings less than 10 m high may be reduced by
20% for stability and design of the framing.
7

5.3.1: Risk Coefficient (k1)
Fig. 1 gives basic wind speeds for terrain
category 2 as applicable at 10 m height above
mean ground level based on 50 years mean
return period. The suggested life span to be
assumed in design and the corresponding k1
factors for different class of structures for the
purpose of design are given in Table 1. In the
design of all buildings and structures, a regional
basic wind speed having a mean return period of
50 years shall be used except as specified in the
note of Table 1.
8

5.3.2: Terrain and Height Factor (k2)
Terrain – Selection of terrain categories shall
be made with due regard to the effect of
obstructions which constitute the ground
surface roughness. The terrain category used in
the design of a structure may vary depending
on the direction of wind under consideration.
Wherever sufficient meteorological information
is available about the wind direction, the
orientation of any building or structure may be
suitably planned.
9

5.3.3: Topography (k3 factor)
The basic wind speed Vb given in Fig. 1 takes
account of the general level of site above sea
level. This does not allow for local topographic
features such as hills, valleys, cliffs, escarpments,
or ridges, which can significantly affect the wind
speed in their vicinity. The effect of topography is
to accelerate wind near the summits of hills or
crests of cliffs, escarpments or ridges and
decelerate the wind in valleys or near the foot of
cliffs, steep escarpments, or ridges.
10

The effect of topography will be significant at a
site when the upwind slope (θ) is greater than
about 3o, and below that, the value of k3 may be
taken to be equal to 1.0. The value of k3 is
confined in the range of 1.0 to 1.36 for slopes
greater than 3o. A method of evaluating the
value of k3 for values greater than 1.0 is given in
Appendix C. It may be noted that the value of k3
varies with height above ground level, at a
maximum near the ground, and reducing to 1.0
at higher levels, for hill slope in excess of 17o.
11

5.3.4: Importance Factor for Cyclonic Region
(k4)
Cyclonic storms usually occur on the east
coast of the country in addition to the Gujarat
coast on the west. Studies of wind speed and
damage to buildings and structures point to
the fact that the speeds given in the basic wind
speed map are often exceeded during the
cyclones. The effect of cyclonic storms is
largely felt in a belt of approximately 60 km
width at the coast.
12

In order to ensure greater safety of structures
in this region (60 km wide on the east coast as
well as on the Gujarat coast), the following
values of k4 are stipulated, as applicable
according to the importance of the structure:
Structures of post–cyclone importance 1.30
Industrial structures 1.15
All other structures 1.00
13

5.4 – Design Wind Pressure
The wind pressure at any height above mean
ground level shall be obtained by the
following relationship between wind pressure
and wind speed:
Pz = 0.6 vz
2
Where,
pz = wind pressure in N/m2 at height z, and
Vz = design wind speed in m/s at height z.
The design wind pressure pd can be obtained
as,
pd = Kd. Ka. Kc. pz
14

Where,
Kd = Wind directionality factor
Ka = Area averaging factor
Kc = Combination factor (See 6.2.3.13)
Note 1 – The coefficient 0.6 (in SI units) in the
above formula depends on a number of factors
and mainly on the atmospheric pressure and air
temperature. The value chosen corresponds to
the average Indian atmospheric conditions.
Note 2 – Ka should be taken as 1.0 when
considering local pressure coefficients.
15

5.4.1 – Wind Directionality Factor, Kd
Considering the randomness in the
directionality of wind and recognizing the fact
that pressure or force coefficients are
determined for specific wind directions, it is
specified that for buildings, solid signs, open
signs, lattice frameworks, and trussed towers
(triangular, square, rectangular) a factor of
0.90 may be used on the design wind pressure.
For circular or near – circular forms this factor
may be taken as 1.0. For the cyclone affected
regions also, the factor Kd shall be taken as 1.0.
16

5.4.2 – Area Averaging Factor, Ka
Pressure coefficients given in Section 6.2 are
a result of averaging the measured pressure
values over a given area. As the area becomes
larger, the correlation of measured values
decrease and vice-versa. The decrease in
pressures due to larger areas may be taken
into account as given in Table 4.
Tributary Area in m2 ≤ 10 Ka =1.0
25 = 0.9
≥ 100 = 0.8
17

6.3 - Force Coefficients:
The value of force coefficients apply to a
building or structure as a whole, and when
multiplied by the effective frontal area Ae of the
building or structure and by design wind
pressure pd, gives the total wind load on that
particular building or structure.
F = Cf Ae pd
Where F is the force acting in a direction
specified in the respective tables and Cf is the
force coefficient for the building.
18

6.3.2.2 – Free standing walls and hoardings
To allow for oblique winds, the design shall
also be checked for net pressure normal to the
surface varying linearly from a maximum of
1.7 Cf at the windward edge to 0.44 Cf at the
leeward edge. The wind load on
appurtenances and supports for hoardings
shall be accounted for separately by using the
appropriate net pressure coefficients.
Allowance shall be made for shielding effects
of one element on another.
19

Lecture 4(02 hours)
Calculate the wind pressure and design
forces on hoarding of 10 m long and 5 m in
height to be fixed at the roof of 24 m high
building near Delhi. The base of the hoarding
board is 2 m above the roof level as shown in
Figure. Also design the hording structures and
hording.
20

Size of hoarding:10 m x and 5 m
Height of roof 24 m
Location: near Delhi.
Base of the hoarding 2 m above the roof
Wind zone IV
Vb = 47 m/s
Terrain category 3 near city area
21

From Table 1 of IS 875
Risk coefficient factor, k1 = 0.71
From Table 2 of IS 875
Terrain and height factor, k2 = 1.05
As per clause 5.3.3 of IS 875
Topography factor, k3 = 1.0
Importance factor for cyclonic region, k4 = 1.0
Design wind speed, Vz = vb x k1 x k2 x k3 x k4
= 35.04 m/s
23

Design wind pressure
pz = 0.6 vz
2
= 0.6 x 35.042
= 736.62 N/m2
 As per clause 5.4.1 of IS 875
kd = 0.9
ka =1
pd = pz x kd x ka
= 0.6633 kN/m2
24

As per the clause 6.3 of IS 875
Wind force on the hoarding, F = Cf Ae pd
Length of the hoarding is 10 m, b/h = 2 and
hoarding is 2 m above roof, using Table 21 of
IS 875, cf = 1.2
 Therefore design wind force on hording is,
F =1.2 x 0.6633 x 1 x 5
= 3.98 kN acting at 4.5 m above the roof
As per the clause 6.3.2.2 of IS 875, to allow
oblique winds
25

Wind force on windward side = 1.7 x 3.98/1.2
= 5.64 kN
Wind force on leeward side = 0.44 x 3.98/1.2
= 1.46 kN
The hoarding will be designed for a wind
pressure of = 1.128 kN/m2.
The frame of hoarding will be designed for
average pressure intensity depending on the
spacing of vertical frame.
26

Design the hoarding structure to support the
hoarding of size 6 x 4 m as shown in Fig.
27

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